The present invention relates to a system and to a method for cooling molds for forming products in expanded polystyrene, particularly expanded polystyrene cores for subsequent use in the production of metal bodies formed by casting by the technique known in English as the xe2x80x9clost foamxe2x80x9d technique.
It is known that, in order to stop the expansion of polystyrene, it is necessary to cool rapidly the walls of the half-molds which are usually made of aluminum and which define the cavity for molding the polystyrene body.
A widely used cooling technique provides for the half-molds to be placed in a depressurized environment and simultaneously struck by jets of water. Since the temperature of the half-molds reaches a value of about 120xc2x0 C. at the moment at which the expansion of the polystyrene is stopped, a fraction of the cooling water changes to the vapor state as soon as it comes into contact with the walls of the mold. The steam thus developed has to be removed quickly in order to shorten the operating cycle of the machine as much as possible.
In order to achieve a high output, it is necessary to produce and maintain a high degree of vacuum in the cooling environment to favor rapid evaporation of the water, enabling the heat of evaporation to be utilized for cooling the mold. The steam produced must also be removed rapidly in order to shorten the cooling-cycle times and hence the molding-cycle times.
Ideally, starting from atmospheric pressure conditions, it would be desirable to re-establish a residual pressure of about 0.2 bars absolute within a maximum period of about 3 seconds at the start of each cooling cycle and to maintain these conditions throughout the cooling time.
Since, in the production of polystyrene elements of the type mentioned above, the amounts of heat to be removed for each cycle are on average about 4000 kcal, which is equal to that absorbed by the evaporation of about 7-8 liters of water, and since it is known that 1 liter of water produces about 1.25 m3 of steam at atmospheric pressure and ambient temperature, about 9 m3 of steam is developed for each cycle and it is therefore necessary to remove steam at a rate of approximately 3 m3 per second.
As will be understood, a system of conventional pumps cannot complete the extraction of such a volume of gas in the times indicated; it would be necessary at least to have a system of extremely high-capacity vacuum pumps, involving high installation and running costs.
Conventional steam extraction systems cannot achieve a degree of vacuum below 0.5-0.6 bars absolute, whereas it would be desirable to have a greater degree of vacuum in order to bring about more rapid evaporation and hence quicker cooling due to the evaporation of the water which, as is known, vaporizes more quickly the lower is the ambient pressure.
The object of the present invention is to provide an improved and highly efficient cooling system which can resolve the limitations of the prior art discussed above. In particular, it is desired to provide a cooling system which can achieve a higher degree of vacuum with the use of conventional suction pumps.
This object is achieved, according to a first aspect of the present invention, by a system for cooling a mold, particularly for expanded polystyrene, by means of jets of water in a depressurized environment, comprising pumping means for depressurizing the environment and removing therefrom the steam generated by the jets of water, wherein said pumping means are arranged to perform the depressurization by suction directly from a steam-condensation device connected to the environment and having means for cyclically discharging the water condensed in said condensation device.
According to another aspect of the present invention, there is proposed a method of cooling a mold, particularly for expanded polystyrene, comprising the steps of:
providing the mold in an environment,
sending jets of cooling water onto the mold,
depressurizing the environment by pumping means and removing from the environment the steam generated by the jets of water,
at least partially recondensing the steam in a condensation device interposed between the environment and the pumping means, and
cyclically discharging the water condensed in the condensation device.